The present invention relates generally to heat exchanger equipment and to processes employing such equipment and more particularly to straining devices which are placed upstream of heat exchangers and other fluid equipment.
One of the most problems associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop by reducing the flow area for the fluid flowing on the inside of the exchanger.
There are a large number of techniques suitable for reducing fouling which can take the form of structural features within the heat exchanger body itself. Significant fouling reduction can however also be achieved by removing debris from the process stream upstream of the heat exchanger. In fact, the presence of debris in various streams that are fed into heat exchangers as well as fluid streams which flow through other devices can cause significant problems that, in some cases, can not be remedied, even by the most effective fouling mitigation technique within the heat exchanger or other device. Fouling can result in problems such as hydraulic limitations, poor heat exchanger thermal performance and premature tube failures causing unplanned unit shutdown. In addition, frequent opening and closing of heat exchangers can lead to poor reliability as a result of wear and tear on the heat exchanger and possible damage to heat exchanger components.
In many petrochemical processes, straining of debris upstream of the heat exchanger is provided by a bucket-type strainer. Unfortunately, because these devices are cumbersome and require frequent cleaning, they are often eliminated from the fluid flow circuits. As a result, many flow streams, although they may have been designed to include a straining function, often do not have one in practice. In some cases, straining functionality may even be left out of the process design because of expense or because of an understanding of the realities of the difficulties in using bucket-type straining devices. Even if one of these straining devices is included in the fluid flow, the strainer must usually be bypassed during cleaning and large debris can therefore pass towards the heat exchanger when cleaning is underway. This problem can be avoided through the use of at least two strainers, connected in parallel, in the process but such a solution adds significant expense. Strainers require isolation, draining, and steam-cleaning before they can be taken apart for cleaning. This is a tedious and time-consuming process.
While various strainer types that provide automatic cleaning as debris builds up within the straining device exist, these devices are generally very expensive, relatively ineffective or both. In addition, since automatic strainers require motors, electrical power is required and the drive mechanism and motor reliability can become concerns. Finally, automatic strainers require a third fluid stream to remove the debris. This stream and the supporting hardware and piping create additional maintenance and upkeep requirements.
According to the present invention, a self-cleaning strainer comprises a movable screen element that is attached to two rollers and placed in the fluid flow path to intercept debris in the fluid comprises:
(a) a housing into which the flow stream passes from the flow pipe for filtration and from which it passes after filtration;
(b) a screen element;
(c) a source roller attached to a first end of the screen element; and
(d) a take-up roller attached to a second end of the screen element.
The screen element extends across the interior of the housing in the path of the flow stream to define (i) a flow region upstream of the screen and a flow region downstream of the screen, (ii) an active portion of the screen element which is in the path of the flow stream and through which the flow stream may pass, and (iii) a non-active portion, and means for rotating the source roller and the take up roller to move the screen element from the source roller to the take-up roller so as to periodically replace the active portion of the screen element with a previously non-active portion of the screen element.
The screen element may be placed perpendicular to the fluid flow direction in the housing so that the fluid flow in a straight line flow path directly through the screen element to continue on through the equipment. Alternatively, the screen element may be placed at another angle to the flow. It may be placed parallel to the general flow axis (but still in the path of the fluid flow), with blocking members to force the flow direction at the inlet to turn in order to pass through the screen. As a result, the fluid passes through the screen element to intercept any debris particles caught up in the fluid.
The rollers contain a length of screen element with its face in the flow channel, that may be rolled from one roller to another over time. The rollers may be operated manually or by an electric motor. Various options are available for triggering the rotation of the rollers to feed new screen element length into the flow channel. In addition to manual rotation as determined by an operator, automated rolling may occur based upon, for example, elapsed time and/or a specific level of debris buildup as measured by an increase in the pressure drop across the screen element. Other automatic advance triggers are also possible.
The present invention provides many advantages including a significant reduction of debris within various fluid flow systems particularly those that include a heat exchanging function. In the case of processes involving heat exchangers, the removal of debris upstream of the heat exchanger provides a very significant amount of fouling reduction and the strainer may be used either with or without other fouling mitigation techniques within the heat exchanger itself. The present strainer may be employed in a great many applications only one of which is processes that include heat exchangers. The present strainer may be used in connection with any application which involves any fluid and which benefits from the removal of debris particles from the fluid flow in order to improve process performance, preserve process equipment or otherwise.